Recent years have seen the interest in the development of renewable energy sources increase as concern over the impact of carbon emissions on the environment has been heightened. Whilst focus has been primarily on the development of wind and solar power, these technologies have various disadvantages. Wind power generation is reliant upon the presence of driving wind of a given threshold value to move the propeller at sufficient speed to drive a turbine. Wind power also requires a large area of land dedicated to the production of energy and these large ‘wind farms’ are often unsightly and may pose a hazard to the surrounding wildlife. Solar power also has the disadvantages of providing a non-reliable source of electricity and also suffers from low efficiency and high cost.
Wave or tidal energy devices can overcome many of the disadvantages listed above. They provide a reliable source of energy as they are driven by the force inherent within tidal and ocean waves and also have the potential to be placed in a large number of areas, particularly in coastal areas with large fetch, such as the western coast of Europe.
A number of differing techniques have been employed to harness wave, tidal or ocean power. Traditional tidal energy devices have centred on a barrier arrangement that when placed within a tidal system fills with water at high tide and releases the water at low tide through a turbine to generate electricity. Concerns have been raised that the use of conventional barrier type tidal energy devices can prove hazardous to wildlife and boats. Additionally, these devices may only be used after each high tide and do not therefore provide a constant supply of energy.
One example of a wave energy collector is disclosed within EP 1115976. This device utilises the relative rotational movement between pluralities of segments to drive a hydraulic motor.
One alternative technique is to use the oscillatory nature of waves to compress a volume of air (an Oscillating Water Column device). By submerging a structure with an air chamber and an underwater aperture, an incident surface wave makes the liquid level within the chamber rise, compressing the volume of air within the air chamber. This (adiabatically) compressed air may then be used to drive a turbine, the rotation of which may be used to power a generator. As the water level falls, the air pressure reduces and air is drawn back into the chamber through the turbine. An example of this type of device is shown within EP 0948716 whereby the parabolic wave is focussed into a chamber wherein the air is compressed and used to drive a unidirectional turbine. Another example of an Oscillating Water Column device has been developed by Wavegen and has been named the ‘Limpet’.
One inherent problem of these devices is the relatively low energy conversion efficiency, coupled to the varying nature of the size and strength of the incident waves, which leads to an uncertain energy output. These devices are also located on or close to the shore to take advantage of the higher parabolic waves at the shore. This again leads to a variation in the production of energy between high and low tides. Additionally, the above devices focus parabolic ocean waves through structural features, for example an upwardly sloped base or a generally upright wall. These devices are also unsuitable in scenarios of constant flow or current, for example tidal flows; thermohaline induced oceanic currents, for example the North Atlantic Drift and the Gulf Stream; and gravity induced liquid flows, for example within rivers.